Hot Swapping the CompactPCI Bus – Design Note 200
Robert Reay
One of the reasons for the increase in popularity of
the CompactPCI bus is its ability to be hot swapped
into and out of a live backplane without turning off the
system power. One of the key elements required for
hot swapping a CompactPCI board is the power supply
isolation controller.
LTC1643 Feature Summary
The LTC1643 features can be summarized as follows:
1. Controls all four CompactPCI supplies: –12V, 12V,
3.3V and 5V.
2. Programmable foldback current limit: a programmable
analog current limit with a value that depends on the
output voltage. If the output is shorted to ground,
the current limit drops to keep power dissipation and
supply glitches to a minimum.
®
The LTC 1643L is designed to meet the power supply
isolation requirements found in the CompactPCI Hot
Swap™ Specification PICMG 2.1. The chip will turn
a board’s supply voltages on and off in a controlled
manner, allowing the board to be safely inserted or
removed without causing glitches on the power supplies and causing other boards in the system to reset.
It also protects against short circuits and reports the
state of the supply voltages via the HEALTHY# signal.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
R2
0.007Ω
1%
MED 5V
R1
0.005Ω
1%
Q1
IRF7413
5V
5A
Q2
IRF7413
3.3V
7.6A
MED 3.3V
R3
10Ω
5%
V(I/O)
R8
1.2k
5%
R9
2k
5%
12V
BD_SEL#
HEALTHY#
9
3VIN
1
2
CompactPCI CONNECTOR
–12V
5
V(I/O)
D2*
12V
C6
0.1μF
R10
2k
5%
R4
10Ω
5%
6
7
C7
0.1μF
10
11
3VSENSE GATE
3
13
3VOUT 5VIN
R5
100Ω
5%
12
5VOUT
12VIN
12VOUT
VEEIN
VEEOUT
ON
LTC1643L
16
15
12V
500mA
–12V
100mA
LONG
3.3V
PRECHARGE OUT
1V ±20%
IOUT = ±40mA
FAULT
PWRGD
C1
0.047μF
14
5VSENSE
GND
TIMER
8
4
C3
0.01μF
C2
0.01μF
D1
1N4148
R11
24
5%
LONG GND
LT1117
R6
130Ω
1%
R7
56.2Ω
1%
V(I/O)
LOCAL_PCI_RESET#
PCI_RST#
DATA LINE EXAMPLE
R14
10Ω, 5%
R13
10k
5%
LONG LONG
3.3V
5V
3.3V
5V
RESET#
DATA BUS
*ISMA12CAT3
DN200 F01
Figure 1. Typical CompactPCI Application
02/99/200_conv
PCI
BRIDGE
(21154)
OUT
IN
ADJ
3. Programmable circuit breaker: if a supply remains
in current limit too long, the circuit breaker will trip,
the supplies will be turned off and the FAULT# pin
will be pulled low.
4. Current limited power-up: the supplies are allowed
to power up in current limit. This allows the chip to
power up boards with large capacitive loads without
tripping the circuit breaker. The maximum power-up
time is programmable using the TIMER pin.
5. –12V and 12V power switches on chip.
ed to the TIMER pin. The current in each pass transistor
increases until it reaches the current limit for that supply. Each supply is then allowed to power up at the rate
dv/dt = 50μA/C1 or as determined by the current limit
and the load capacitance, whichever is slower. Currentlimit faults are ignored while the TIMER pin voltage is
ramping up. Once all four supply voltages are within
tolerance, the HEALTHY# signal will pull low.
BD_SEL#
10V/DIV
6. BD_SEL# and HEALTHY# signals.
7. Space-saving 16-pin SSOP package.
Typical Application
Figure 1 shows a typical application using the LTC1643L.
The power supplies are controlled by placing external
N-channel pass transistors Q1 and Q2 in the 5V and
3.3V power paths. Internal pass transistors control
the 12V and –12V power paths. Resistors R1 and R2
sense overcurrent conditions and R5 and C1 provide
current control loop compensation. Resistors R3 and
R4 prevent high frequency oscillations in Q1 and Q2.
Capacitor C2 provides the power-up timing and C6 and
C7 provide chip bypassing on the 12V and –12V inputs.
Diode D2 protects the part from voltage surges below
–15V on the –12V supply.
The 3.3V, 5V, 12V and –12V inputs of the LTC1643 come
from the medium length power pins on the CompactPCI
connector. The long 3.3V, 5V and V(I/O) pins power up
the bus-precharge circuit, the PCI bridge chip and the
LOCAL_PCI_RESET# logic. The BD_SEL# signal is
connected to the ON pin and the HEALTHY# signal is
connected to the PWRGD pin. The HEALTHY# signal
is combined with the PCI_RESET# signal to generate
the LOCAL_PCI_RESET# signal.
The 1V precharge voltage for the data bus lines is generated by an LT®1117 low dropout regulator. The output
of the LT1117 is set to 1.8V, then the voltage is dropped
by an 1N4148 diode to generate 1.0V. The precharge
circuit is capable of sourcing and sinking 40mA.
TIMER
10V/DIV
GATE
10V/DIV
12VOUT
5V/DIV
5VOUT
5V/DIV
3VOUT
5V/DIV
VEE(OUT)
5V/DIV
PRECHARGE
2V/DIV
HEALTHY#
10V/DIV
20ms/DIV
DN200 F02
Figure 2. Normal Power-Up
Conclusion
Using the LTC1643L, a CompactPCI board can be made
hot swappable so the system power can remain on when
the board is inserted or removed. With the LTC1643L,
safe hot-swapping becomes as easy as hooking up an
IC, a couple of power FETs and a handful of resistors
and capacitors.
Power-Up Sequence
Figure 2 shows a typical power-up sequence.
When the BD_SEL# is pulled low, the pass transistors are
allowed to turn on and a 20μA current source is connectData Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn200f_conv LT/TP 0299 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1999